Considerable growth in the knowledge and technology of cryogenics has occurred during the past three decades. For example, see Haselden, Cryogenic Fundamentals, published 1971 by Academic Press, Longon; Bailey, Advanced Cryogenics, published 1971 by Plenum Press, London; Barron, Cryogenic Systems, published 1966 by McGraw-Hill, Inc., New York; Cryogenic Engineering (Proceedings of the First Cryogenic Engineering Conference in Japan on April 9-13, 1967), published 1968 by Heywood Temple Industrial Publications Ltd., London; and U.S. Air Force Technical Report AFFDL-TR-73-149 Volume 1, December 1973 (Proceedings of the Cryogenic Cooler Conference at USAF Academy, Colorado, on October 16-17, 1973). Nevertheless, the capability and efficiency with which a variety of cryogenic equipment can operate to produce and maintain service temperatures below about 15.degree. K. (and as low as about 2.degree. K. or less) has still been limited and hampered by the availability of materials that have adequate combinations of thermal and/or dielectric and/or mechanical properties and are economical to manufacture in the required form of apparatus elements and components.
Heat exchange means in a cryogenic refrigeration system with fluid refrigerant is one of the most important elements in determining the capability and efficiency of the system (see Cryogenic Engineering, supra, p. 222). In such a system utilizing a regenerator as the heat exchange means, the capability and efficiency are importantly governed by the thermal properties of the regenerator packing material (see Bailey, supra, p. 200-1, and Barron, supra, p. 321). One such thermal property that is very important and intrinsic to the packing material is the volumetric heat capacity (or specific heat) of that material at intended service temperature. Heretofore, Pb shot or other shaped bodies of lead or lead-antimony alloys (or even copper) have commonly been employed as the packing material giving the then best combination of specific heat and economical manufacture for systems operating as low as about 15.degree. K. or lower. Nevertheless, these common packing materials present difficulties in maintaining significant thermal loads at temperatures in the range of about 6.degree.-15.degree. K. with good efficiency and without complex multicycle systems or equipment. Of course, there have been suggestions to fabricate regenerator structures of other less economical materials with specific heats higher than that of lead at such very low temperatures, viz. materials with "excess" specific heat (added to their basic Debye contribution) due to phenomena like phase, magnetic and order-disorder transformations within the materials at low temperature, such as neodymium with the additional specific heat contribution of its magnetic transformation (see Haselden, supra, p. 314), or, as more simply and customarily stated, with specific heat anomalies (see Bailey, supra, p. 201).
I perceive that such materials with the higher specific heat at very low temperatures can also be used advantageously to fabricate other thermal energy absorbing elements in cryogenic refrigeration systems (including those with solid refrigerants such as paraelectric refrigerants--see U.S. Pat. No. 3,638,440) so as to enhance the capability and efficiency of those other elements and of the systems as a whole. Among such other elements are those which serve as thermal dampers and thermal isolators. If such materials were additionally characterized by being dielectrics, they could advantageously serve also as dielectric insulation and dielectric isolators of superconducting components operative at the very low temperatures.